Laser cutting method and laser cutter

ABSTRACT

A development of a laser cutting method and a laser cutter in which the adverse effects such as self burning to the cutting surface can be prevented, when the material is cut at a high speed, has been desired. In order to achieve the object, the present invention provides a laser cutting method and a laser cutter in which a cutting laser beam  22  is applied to a material  25  to be cut at a cutting point  26,  while ejecting gases  23  and  24  to the cutting point  26  or its surrounding area from a plurality of nozzle openings  30  and  31  arranged in a ring or in a line, wherein the oxygen concentrations of the gases  23  and  24  ejecting from at least one of the nozzle opening  30  and  31  are changed to adjust the oxygen concentration distribution in an area within several millimeters of the cutting point  26;  thereby the cutting quality can be improved.

TECHNICAL FIELD

The present invention relates to a laser cutting method and a lasercutter.

BACKGROUND ART

When mild steel is cut with a laser having a small power relative to thethickness of the mild steel, sufficient cutting laser energy can beobtained at the vicinity of the surface of the mild steel at which thelaser is applied. However, there are many cases in which sufficientcutting laser energy cannot arrive at the inside of the mild steel.Therefore, in recent years, high concentration (purity) oxygen (having aconcentration (purity) of 99.5% or more) is supplied to the vicinity ofthe cutting point by use a co-axial double nozzle 1 a and 1 b as shownin FIGS. 10 and 11, a co-axial triple nozzle (not shown in Figures), oran auxiliary nozzle 2 as shown in FIG. 12. Thereby, a sufficient oxygenconcentration for cutting the inside of the mild steel is maintained.Heat is generated due to the oxidation reaction between steel and thesupplied oxygen, and the heat compensates for the deficiency of laserenergy of the laser cutter.

In the laser cutting method using the co-axial double nozzle 1 or theauxiliary nozzle 2, only the oxygen quantity is adjusted, and the oxygenconcentration is not adjusted. Therefore, it is difficult to maintain asuitable cutting quality when the thickness or the kinds of the materialto be cut is changed. An excess combustion heat is generated due to thesmall change in conditions when the material is cut, and easilyadversely effects to a quality of the cut surface.

That is, when the mild steel is cut at a high speed (1.5 m/min orgreater), a high temperature area 3 (represented by a two-dot dashedline in FIGS. 13 and 14) suitable for combustion between oxygen andsteel is generated at the cutting surface behind the cutting point in acutting direction, rather than the forward to the cutting point in acutting direction by the influence of the heat conductivity and thethermal diffusivity, as shown in FIGS. 13 and 14. Then excess combustionenergy adversely effects to the cutting surface and the cutting quality.Consequently, for example, as shown in FIG. 15, a normal cutting qualitycan be obtained at the vicinity of the surface 4 of the material to becut; however, an abnormal cutting quality is often obtained on theinside of the material to be cut. In addition, a self burningphenomenon, dross adhesion, or cutting notch generation easily occurs onthe inside of the material. Therefore, it is impossible to sufficientlyimprove the cutting quality. When the laser beam pulse is outputcontinuously in order to cut the material at a high speed, the frequencyof occurrence of the self burning phenomenon increases. Therefore, inorder to prevent the occurrence of the self burning phenomenon, usuallythe laser beam pulse is intermittently output. However, when the laserbeam pulse is intermittently output, the cutting speed decreases.

When the laser cutter applying a laser beam 5 and a cutting gas (highconcentration oxygen) 6 and 6 from the same nozzle shown in FIGS. 13 and14 is used, the cutting gas 6 and 6 diffuses more widely than thediameter of the laser beam 5. The gas stream of the cutting gas 6consequently generates behind the cutting point in a cutting direction.When the diameter of the nozzle is set so as to equal the diameter ofthe laser beam, the combustion energy at the high temperaturedistributional area formed behind the cutting point in the cuttingdirection, increases due to oxygen. Therefore, it is impossible to solvethe problem. The cutting quality depends on the position of the centerof the laser beam 5. In order to adjust the proper diameter of thenozzle against laser beam diameter 5, it is necessary to frequentlychange the nozzle.

DISCLOSURE OF INVENTION

Therefore, the object of the present invention is to provide a lasercutting method having the following effects:

(a) the oxygen concentration distribution can be easily changed in anarea within several millimeters of the cutting point by changing theoxygen concentration of the gas ejecting from a plurality of nozzles;thereby the cutting quality can be improved,

(b) the adverse effects to the cutting surface such as self burningphenomenon, can be prevent by decreasing the oxygen concentration behindthe cutting point in a cutting direction; thereby a high cutting qualitycan be obtained, and

(c) these effects can be obtained; thereby a high cutting quality can beobtained even when the material is cut at a high speed.

In addition, the object of the present invention is to provide a lasercutter having the following effects:

(d) gas supplying means having an oxygen concentration changing functioncan respectively eject a gas containing a required amount of oxygen, andthe oxygen concentration distribution can be easily changed in an areawithin several millimeters of the cutting point; thereby a high cuttingquality can be easily obtained, and

(e) an oxygen concentration distribution can be easily and exactlychanged by a control device for controlling the mass flow rate of theejecting gas; thereby the cutting quality can be improved.

The laser cutting method of the present invention comprises the step ofapplying a cutting laser beam to a material to be cut at a cutting pointwhile ejecting gas to the cutting point or its surrounding area from aplurality of nozzles arranged in a ring or in a line, wherein the oxygenconcentration of the gas ejecting from at least one of the nozzles ischanged to adjust the oxygen concentration distribution in an areawithin several millimeters of the cutting point. It is possible toobtain an oxygen concentration suitable for laser cutting at the cuttingpoint and another oxygen concentration suitable for preventing thedisadvantages such as the self burning phenomenon in the areasurrounding the cutting point; therefore, a high cutting quality can beobtained by the laser cutting method of the present invention.

Many oxygen concentration distributions can be obtained in the lasercutting method of the present invention. In order to obtain a highcutting quality, the most suitable oxygen concentration distribution isformed depending on the cutting conditions. Moreover, even while thematerial is being cut, it is possible to change the oxygen concentrationdistribution.

The oxygen concentration distribution preferably comprises a high oxygenconcentration at the vicinity of the cutting point and a low oxygenconcentration behind the cutting point in a cutting direction. Theoxygen concentration distribution can be formed by ejecting the gascontaining oxygen at a high concentration from the at least one nozzle,and ejecting the other gas containing oxygen at a low concentration.Thereby, the high oxygen concentration (having a concentration of 99.5%or more) suitable for a laser cutting can be maintained in the vicinityof the cutting point. Moreover, the other oxygen concentration suitablefor preventing the combustion between oxygen and steel can be maintainedbehind the cutting point in a cutting direction, and the generation ofthe self burning phenomenon, dross adhesion, or the like can beprevented.

In order to prevent the generation of the self burning phenomenon andthe like, it is preferable to set the oxygen concentration at thedownstream in an ejecting direction of the gas stream low. However, itis more preferable to maintain the oxygen concentration ensuring thelaser cutting at the downstream in the ejecting direction of the gasstream.

In addition, the laser cutting method of the present invention has thefollowing effects:

(1) a high cutting quality can be obtained even when a laser beamcomprising a continuous pulse is used; therefore, a cutting speed can beimproved,

(2) it is possible to adjust the variation of the cutting conditionssuch as a thickness of the material to be cut by changing the oxygenconcentration or the flow rate of the gas containing oxygen at a lowconcentration and the gas containing oxygen at a high concentration;therefore, an exchanging the nozzle is not necessary and the cuttingefficiency can be improved, and

(3) a melting material generated at the cutting point can be easilyremoved; therefore, the cutting quality at the cutting point can be alsoimproved.

The aforementioned “a plurality of nozzle openings” denotes a pluralityof nozzle openings formed in one nozzle, a plurality of nozzle openingscomprising a plurality of nozzles, a plurality of nozzle openingincluding auxiliary nozzle openings, and a plurality of nozzle openingsassembled these nozzle openings. The nozzle may not eject the gasparallel to the laser beam. Some nozzles preferably eject the gas at anangle to the laser beam. When the nozzles eject the gas at an angle thelaser beam, the required oxygen concentration can be easily adjusted.Moreover, it is more preferable to randomly arrange the nozzles eachother so as to eject the gas to the different directions. Thereby, therequired oxygen concentration can be more easily adjusted.

A laser cutter of the present invention comprises a plurality of nozzlesarranged in a ring or in a line, and gas supplying means for supplyingthe gas to the nozzles and changing the oxygen concentration of the gas,wherein the oxygen concentration of the gas ejecting from the nozzles ischanged by the gas supplying means. Therefore, the oxygen concentrationdistribution required in the laser cutting method of the presentinvention is easily adjusted. Moreover, when the cutting conditions suchas the thickness of the material to be cut are changed, the requiredoxygen concentration distribution can be easily prepared by changing thegas quantity using the gas supplying means. Thereby, the cutting qualitycan be easily improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the preferred laser cutter according to the presentinvention.

FIG. 2 shows the first preferred embodiment of the laser cutting methodaccording to the present invention, and is a planar view showing thevicinity of the cutting point when the co-axial double nozzle is used.

FIG. 3 is a sectional view of FIG. 2, and shows the oxygen concentrationdistribution prepared by the high concentration oxygen and the lowconcentration oxygen ejecting from the co-axial double nozzle.

FIG. 4 shows the effects obtained from the first preferred embodiment ofthe laser cutting method according to the present invention, and showsthe roughness of the cutting surface with respect to the distance fromthe top surface when mild steel having a thickness of 12 mm is cut.

FIG. 5 shows the effects obtained from the first embodiment of the lasercutting method according to the present invention, and shows theroughness of the cutting surface with respect to the distance from thetop surface when mild steel having a thickness of 22 mm is cut.

FIG. 6 shows the nozzle used in the first embodiment of the lasercutting method according to the present invention, and is a bottom viewshowing the nozzle comprising a center nozzle and a plurality of nozzlesarranged in a ring so as to enclose the center nozzle.

FIG. 7 shows the second embodiment of the laser cutting method accordingto the present invention, and is a sectional view showing the oxygenconcentration distribution formed at the vicinity of the cutting point.

FIG. 8 shows the second embodiment of the laser cutting method accordingto the present invention, and is a bottom view showing the nozzlecomprising three nozzle openings arranged in a line.

FIG. 9 shows the effects obtained from the laser cutting method of thepresent invention, and is a perspective view showing the cutting surfaceformed by the laser cutting method of the present invention.

FIG. 10 is a sectional view showing the co-axial double nozzle.

FIG. 11 is a bottom view showing the co-axial double nozzle shown inFIG. 10.

FIG. 12 shows the auxiliary nozzle.

FIG. 13 shows the problems to be solved of the present invention, and isa plan view showing the vicinity of the cutting point when the co-axialdouble nozzle is used.

FIG. 14 is a sectional view showing the co-axial double nozzle shown inFIG. 13.

FIG. 15 shows the problems to be solved of the present invention, and isa perspective view showing the cutting surface formed by theconventional laser cutting method.

MODES FOR CARRYING OUT THE INVENTION

Hereinbelow, embodiments of the present invention will be explained indetail with reference to FIGS. 1 to 9.

FIG. 1 shows one embodiment of the laser cutter 10 of the presentinvention.

The laser cutter 10 shown in FIG. 1 comprises a laser beam oscillator 10a, a nozzle 11, gas supplying means (a gas supplying apparatus) 12 a and12 b for supplying gas to the nozzle 11. The nozzle 11 further comprisesopenings 11 a and 11 b. The gases supplied from the gas supplying means(gas supplying apparatuses) 12 a and 12 b eject from the openings 11 aand 11 b. That is, the nozzle 11 comprises the opening 11 a for ejectingthe gas supplied from the gas supplying means 12 a and the other opening11 b for ejecting the gas supplied from the other gas supplying means 12b.

The high concentration oxygen having a concentration of 99.5% or moresupplied from the oxygen resource 13 and a gas other than oxygensupplied from the gas resource 14 are mixed in the mixer 15. The mixer15 supplies the obtained mixed gas containing the required amount ofoxygen to the nozzle opening 11 a. The oxygen supplied from the oxygenresource 13 flows into the mixer 15, via the concentration meter 16 andthe mass flow controller (MFC) 17. The measuring signal obtained in theconcentration meter 16 and the mass flow controller 17 is transmitted tothe concentration controller 18. The mass flow rate of oxygen flowinginto the mixer 15 is exactly calculated in the concentration controller18. The mass flow controller 17 functions as a mass flow rate meter foroxygen, and controls the mass flow rate of oxygen flowing into the mixer15.

The concentration controller 18 connects the mass flow controller 19arranged in the gas passage between the gas resource 14 and the mixer15, and outputs the driving signal to the mass flow controller 19;thereby, the concentration controller 18 controls the flow rate of thegas flowing into the mixer 15.

Moreover, many kinds of gas can be used as a gas supplied from the gasresource 14. For example, noncombustible gases such as nitrogen, argon,and carbon dioxide, and combustible gases such as compressed air,hydrogen, methane can be used. However, when compressed air is used, itis necessary to remove the impurities such as oil mist.

The concentration controller 18 connects the input means 20 and the datatable 21. The cutting conditions such as a kind and a thickness ofmaterial to be cut, and a cutting speed are input to the concentrationcontroller 18 by the input means 20. The data table 21 accumulates theoptimum oxygen concentrations depending on these cutting conditions. Theconcentration controller 18 selects the optimum oxygen concentrationfrom the data table 21 to adjust the inputs from the input means 20.Then the concentration controller 18 controls the mass flow controller19 based on the inputs, and adjusts the oxygen concentration and theflow rate of the mixed gas supplied to the nozzle opening 11 a from themixer 15.

The other gas supplying means 12 b connecting to the nozzle opening 11 bcomprises the same structure as the gas supplying means 12 a, andsupplies the mixed gas containing the required oxygen concentration atthe required flow rate from the mixer 15 to the nozzle opening 11 b.These gas supplying means 12 a and 12 b connect to the sameconcentration controller 18. The concentration controller 18 relativelyadjusts the flow rate and the oxygen concentration of the mixed gasessupplied from the gas supplying means 12 a and 12 b. The data table 21accumulates also the data for relatively adjusting the flow rate and theoxygen concentration of the mixed gases supplied from the gas supplyingmeans 12 a and 12 b.

Moreover, the laser cutter of this embodiment can comprise the nozzlecomprising three or more openings, or three or more gas supplying means.

The oxygen resource 13 usually supplies the oxygen having a requiredconcentration and no impurities; therefore, it is possible to omit theconcentration meter 16. When the concentration meter 16 is omitted, theoxygen concentration of the oxygen supplying to the mass flow controller17 is adjusted in the oxygen resource 13, depending on the cuttingconditions.

Hereinbelow, embodiments of the laser cutting method according to thepresent invention will be explained in detail.

The first embodiment of the laser cutting method will be explainedbelow.

FIG. 2 is a planar view showing the co-axial double nozzle, for example,shown in FIG. 6. In FIG. 2, reference numeral 22 denotes a laser beam,reference numeral 23 denotes a gas containing oxygen at a highconcentration (a concentration of 99.5% or more), reference numeral 24denotes a gas containing oxygen at a low concentration, and referencenumeral 25 denotes a material to be cut (steel plate).

The diameter of the laser beam 22 is in a range from 0.1 to 0.5 mm. Thegas steam of the high concentration oxygen 23 diffuses at the areawithin approximately 1.5 mm of the laser beam 22. Furthermore, the gasstream of a mixed gas 24 diffuses to the area surrounding the gas streamof the high concentration oxygen 23. The mixed gas 24 comprises oxygenand the gas other than oxygen such as nitrogen. The oxygen concentrationof the mixed gas 24 is lower than that of the high concentration oxygen23.

The high concentration oxygen 23 and the mixed gas 24 can be easilyformed by the laser cutter 10 shown in FIG. 1. Moreover, the highconcentration oxygen 23 and the mixed gas 24 respectively denote the gascontaining oxygen at a high concentration and the gas containing oxygenat a low concentration in Claim 2.

FIG. 3 is a sectional view showing the oxygen concentration distributionat the vicinity of the cutting point 26 made by applying the laser beam22 at the surface of the material 25 to be cut.

As shown in FIG. 3, the laser beam 22 straightly penetrates through theflat material 25 to be cut (steel plate). The gas stream of the highconcentration oxygen 23 maintains the fixed diameter from the nozzleopening to the top surface of the material 25 to be cut; however, thegas stream of the high concentration oxygen 23 diffuses in the cut hole27. The gas stream of the mixed gas 24 ejects parallel to the gas streamof the high concentration oxygen 23, flows into the cut hole 27, anddiffuses to the center of the gas stream of the high concentrationoxygen 23 at the bottom of the cut hole 27. The gas stream of the highconcentration oxygen 23 and the gas stream of the mixed gas 24 arediffused and mixed at the bottom of the cut hole 27. Thereby, theintermediate oxygen concentration area 28 having an oxygen concentrationintermediate between the oxygen concentration of the high concentrationoxygen 23 and the mixed gas 24, is formed.

Moreover, in FIG. 3, reference numeral 29 denotes a nozzle comprisingco-axial double nozzle openings 30 and 31. The high concentration oxygen23 ejects from the inner nozzle opening 30. The mixed gas 24 ejects fromthe outer nozzle opening 31.

According to the cutting method of the first embodiment, the material iscut in the high concentration oxygen 23. The combustion energy for ironand oxygen decreases behind the cutting point in the moving direction ofthe laser beam 22 indicated by the an arrow in FIG. 3. Therefore, thegeneration of the self burning phenomenon, dross adhesion, or cuttingnotch can be prevented, and adverse effects to the cutting surface (wallof the cut hole 27) can be prevented. That is, the mixed gas 24 having alow oxygen concentration covers behind the high concentration oxygen 23in a cutting direction, as shown in FIG. 2. Therefore, the combustionenergy for steel and oxygen at the cutting surface significantlydecreases, the adverse effects due to the excess combustion energy tothe cutting surface can be prevented, and the smooth cutting surface canbe obtained.

The oxygen concentration so as not to obstruct the cutting the materialis maintained at the bottom of the cut hole 27 by the formation of theintermediate oxygen concentration area 28, as shown in FIG. 3.Therefore, the generation of the self burning phenomenon, drossadhesion, or cutting notch can be prevented. A smooth cutting surfacehaving no abnormalities shown in FIG. 9, can be obtained.

The generation of the melting material at the cutting point 26 can beeasily prevented and the cutting quality at the cutting point 26 isimproved by preventing the adverse phenomena such as self burning behindthe cutting point 26. When the material is cut by a laser beamcomprising a continuous pulse at a high speed, the laser beam does notadversely influence to the cutting surface. Therefore, the cutting speedincreases. In addition, even when the material is cut at a high speed, ahigh cutting quality can be obtained.

It is possible that the diameter of the laser beam 22 corresponds to thevariation of the cutting conditions such as a thickness of the materialto be cut by only changing the diameter of the gas stream of the highconcentration oxygen 23 and the gas stream of the mixed gas 24, or theoxygen concentration of the mixed gas 24. Therefore, in order to adjustthe diameter of the laser beam, exchanging the nozzle is not necessary.Thus, the cutting efficiency can be improved. When the laser cuttershown in FIG. 1 is used, the diameter of the gas streams can be easilyadjusted by changing the mass flow rate of the high concentration oxygen23 and the mixed gas 24. Therefore, the cutting efficiency can also beimproved.

The laser cutter in which the gas having a required oxygen concentrationdirectly supply to the nozzle openings 30 and 31 can also be used in thelaser cutting method of this embodiment, in addition to the laser cutter10 shown in FIG. 1.

FIGS. 4 and 5 show the roughness Rz (μm) of the cutting surface when thelaser cutting method of this embodiment according to the presentinvention and the conventional laser cutting method were performed underthe same cutting conditions.

In FIG. 4, the laser beam output is 6 kW, the laser beam comprising acontinuous pulse is used, the thickness of the material (steel plate:black mild steel) to be cut is 12 mm. In the laser cutting method (shownin FIG. 3) of this embodiment, the oxygen having an oxygen concentrationof 99.8% is supplied from the inner nozzle opening 30, and the mixed gashaving an oxygen concentration of 84.0% is supplied from the outernozzle opening 31. In the conventional laser cutting method, oxygenhaving an oxygen concentration of 99.8% is supplied from the inner andouter nozzle openings 30 and 31.

The laser cutting conditions of FIG. 5 are the same as those of thelaser cutting conditions of FIG. 4, except that the thickness of thematerial to be cut is 22 mm.

As shown in FIGS. 4 and 5, the roughness Rz of the cutting surfaceformed by the laser cutting method of this embodiment is smaller thanthe roughness Rz of the cutting surface formed by the conventional lasercutting method, in both cases in which the thickness of the material tobe cut is 12 mm and 22 mm. As shown in FIG. 4, when the thickness of thematerial to be cut is 12 mm, the roughness Rz of the cutting surface atthe point about 3 mm away from the top surface is remarkably improved.In addition, as shown in FIG. 5, when the thickness of the material tobe cut is 22 mm, the roughness Rz at every point of the cutting surfaceis remarkably improved.

Therefore, the cutting quality is improved by forming the optimum oxygenconcentration distribution in the cut hole in the laser cutting methodof the present invention. The excellent effects which cannot be obtainedby the conventional laser cutting method in which the mixed gases havinga uniform oxygen concentration are ejected from the nozzle openings, canbe obtained by the laser cutting method of the present invention.

Moreover, it is necessary that the ejecting conditions of the highconcentration oxygen 23 and the mixed gas 24 correspond to the cuttingconditions, in order to always obtain a high cutting quality.

The present inventor has proved that the oxygen concentration of thehigh concentration oxygen 23 supplied from the inner nozzle opening 30is preferably 99.5% or more.

When the mild steel is cut under conditions in which the oxygenconcentration of the high concentration oxygen is 99.5% or more, twokinds gas are used, that is, the high concentration oxygen and the mixedgas are used; a laser beam comprising a continuous pulse is used, theoutput of the laser beam is 6 kW, and the thickness of the mild steel is9 mm, then the oxygen concentration of the mixed gas is preferably 70%or less. In this case, even when the oxygen concentration of the mixedgas is 0%, it is possible to prevent the adverse effects due to theexcess combustion energy to the cutting surface; therefore, a highercutting quality than the cutting quality obtained by the conventionallaser cutting method can be obtained. For example, when the mild steelhaving a thickness in a range from 16 to 20 mm is cut under the abovecutting conditions, the practical oxygen concentration of the mixed gasis 50% or more. When the mild steel is cut under cutting conditions inwhich the oxygen concentration of the mixed gas is less than 50%,significant the dross adhesion occurs and the cutting quality decreases.Therefore, such cutting conditions are not suitable for practical use.When the thickness of the mild steel is 15 mm or more and the oxygenconcentration of the mixed gas is too small, the disadvantages such assignificant dross adhesion often arises. However, it is possible toprevent the arising of the disadvantages by maintaining the lower limitof the oxygen concentration of the mixed gas at 50%. The higher limit ofthe oxygen concentration of the mixed gas is approximately 90%. When thehigher limit of the oxygen concentration of the mixed gas is more than90%, it is difficult to prevent these disadvantages from arising. Whenthe thickness of the mild steel is 25 mm, the practical lower limit ofthe oxygen concentration of the mixed gas is 70%.

Many nozzles having a different structure can be used in the lasercutting method of this embodiment.

The high concentration oxygen 23 and the mixed gas 24 shown in FIGS. 2and 3 can be obtained by using an auxiliary nozzle A plurality ofauxiliary nozzles can be used.

FIG. 6 is a bottom view showing one nozzle.

The nozzle 32 shown in FIG. 6 comprises the center nozzle opening 33 andthree outer nozzle openings 34 enclosing the center nozzle opening 33 ina ring line. The high concentration oxygen is ejected from the centernozzle opening 33, and the mixed gas having a low oxygen concentrationis ejected from each the outer nozzle openings 34. When the nozzle 32 isused in the laser cutting method of the present invention, it ispreferable to arrange one outer nozzle opening 34 in front of thecutting point in the cutting direction, and two other outer nozzleopenings 34 and 34 behind the cutting point in the cutting direction.Thereby, the cutting quality can be improved more efficiently. That is,as shown in FIGS. 2 and 3, it is particularly important to decrease theoxygen concentration beside and behind the cutting point 26. The area inwhich the oxygen concentration is low can be formed by ejecting themixed gas from the outer nozzle openings 34 and 34 arranged behind thecutting point in the cutting direction. When the nozzle 32 is used, itis possible to decrease reliably the oxygen concentration beside andbehind the cutting point 26; therefore, the cutting quality can bereliably improved. In addition, the mixed gas ejected from the outernozzle openings 34 does not entirely encircle the gas stream of the highconcentration oxygen; therefore, the amount of the mixed gas used can bedecreased The cost for laser cutting can decreases.

Moreover, this embodiment adopted a nozzle comprising three outer nozzleopenings, but the nozzle is not restricted to 3 outer nozzle openings.It is also possible to arrange the outer nozzle openings so as to have adifferent opening shape around the center opening 33.

It is further possible to eject the gases having a different oxygenconcentration from a plurality of the outer nozzle openings surroundingthe center nozzle opening 33. Thereby, it is possible to efficientlyadjust the oxygen concentration distribution around the cutting point;therefore, a high cutting quality can be obtained.

Hereinbelow, the second embodiment of the laser cutting method accordingto the present invention will be explained in detail with reference toFIGS. 7 and 8.

As shown in FIG. 7, in this embodiment of the laser cutting method, twothe gas streams 35 and 36 having a high oxygen concentration of 99.5%and the mixed gas stream 37 comprising oxygen and the gas other thanoxygen such as nitrogen are ejected from the nozzle 40 to the vicinityof the cutting point 39 a where the laser beam 39 is applied to thematerial 38 (steel plate) to be cut. As shown in FIGS. 7 and 8, thenozzle 40 comprises three nozzle openings 41, 42, and 43 arranged in aline. The high concentration oxygen is ejected from the nozzle opening41 arranged in the front in the cutting direction and the nozzle opening43 arranged in the rear in the cutting direction, and the mixed gas 37is ejected from the center nozzle opening 42.

Moreover, the high concentration oxygen 35 and 36 and the mixed gas 37respectively denote the gas containing oxygen at a high concentrationand the gas containing oxygen at a low concentration in Claim 2.

The high concentration oxygen 35 and 36, and the mixed gas 37 arerespectively ejected from the nozzle 40, and form the gas streams havinga diameter of approximately 1.5 mm. The gas stream comprising the highconcentration oxygen gas 35 ejected from the nozzle opening 41 arrangedin the front of the cutting direction penetrates through the cut hole 44formed in the material 38 to be cut, while the gas stream comprising thehigh concentration oxygen gas 35 encloses the laser beam 39. The mixedgas 37 forms the oblique gas stream ejecting behind the gas stream ofthe high concentration oxygen 35 in the cutting direction. The gasstream of the mixed gas 37 interflows with the gas stream comprising thehigh concentration oxygen 35 at the portion at which from the center tothe bottom (bottom in FIG. 7) in the thickness direction of the material38 to be cut in the cut hole 44. The high concentration oxygen 36 formsthe oblique gas stream ejecting behind the gas stream of the mixed gas37 in the cutting direction. The gas stream comprising the highconcentration oxygen 36 interflows with the gas stream of highconcentration oxygen 35 below the portion where the gas stream of thehigh concentration oxygen 35 and the gas stream of the mixed gas 37interflow.

The required laser cutting can be performed in the high concentrationgas 35 at the upside of the cut hole 44. Behind the cutting point 39 ain the cutting direction, the combustion energy for iron and oxygendecreases due to the existence of the mixed gas 37. Therefore, thegeneration of self burning and dross adhesion can be prevented. Belowthe cutting point 39 a in the thickness direction of the material 38 tobe cut, the high concentration oxygen 35 and the mixed gas 37 are mixed,the intermediate oxygen concentration area 45 having an oxygenconcentration between the oxygen concentration of the high concentrationoxygen 35 and the mixed gas 37, is formed. The laser cutting can beperformed in this area; but the adverse effects such as self burning canbe prevented. Below the intermediate oxygen concentration area 45 in thethickness direction of the material 38 to be cut, the stable oxygenconcentration area 46 is formed. The high concentration oxygen 35 and36, and the mixed gas 37 diffuses, and are mixed; thereby the stableoxygen concentration area 46 is formed. The oxygen concentration at thestable oxygen concentration area 46 substantially equals to the oxygenconcentration at the intermediate oxygen concentration area 45. Behindthe intermediate oxygen concentration area 45 in the cutting direction,the oxygen concentration increases due to the high concentration oxygen36. However, the intermediate oxygen concentration area 45 is separatedfrom the cutting point 39 a; therefore, the temperature at theintermediate oxygen concentration area 45 is low, and the combustionenergy for iron and oxygen decreases. The adverse effects such as selfburning do not occur.

Therefore, according to the laser cutting method of this embodiment itis possible to prevent adverse effects such as self burning whilemaintaining the cutting, due to the oxygen concentration distributionformed at the vicinity of the cutting point 39 a. Consequently, thecutting quality can be improved. In addition, even when the thickness ofthe material to be cut is large, sufficient combustion energy for ironand oxygen can be maintained due to the stable oxygen concentration area46 formed by the high concentration oxygen 37 below the intermediateoxygen concentration area 45. Therefore, a high cutting quality can beobtained.

Moreover, according to the laser cutting method of this embodiment, itis possible to easily adjust the position of the stable oxygenconcentration area 46 and the combustion energy distribution in thedirection of the thickness of the material to be cut by changing theflow rate of the high concentration oxygen 37. Even when the laser beamenergy is small, the combustion energy at the stable oxygenconcentration area 46 can be decreased by increasing the mass flow rateof the high concentration oxygen 37; therefore, excellent cuttingproperties can be maintained. In this case, the high concentrationoxygen 37 does not influence to the original oxygen concentration of thecutting point 39 a and the intermediate oxygen concentration area 45;therefore, the cutting property at the cutting point 39 a and theintermediate oxygen concentration area 45 can be maintained. Thecombustion energy distribution can be easily adjusted, and the cuttingquality can be easily improved. Moreover, the material 38 to be cuthaving a large thickness can be efficiently cut due to the spread of thestable oxygen concentration area 46 by increasing the mass flow rate ofthe high concentration oxygen 37.

The present invention is not limited to the embodiments explained above,but the structure of the nozzles in the embodiments can be variouslymodified within the scope of the claimed invention.

Moreover, according to the laser cutting method of the presentinvention, when the co-axial double nozzle is used, it is possible toeject the gas having a low oxygen concentration from the inner nozzleopening, and the gas having a high oxygen concentration from the outernozzle opening.

What is claimed is:
 1. A laser cutting method in which a cutting laserbeam is applied to a material to be cut at a cutting point, whileejecting gas to the cutting point or its surrounding area from aplurality of nozzles arranged in a ring or in a line, wherein an oxygenconcentration of the gas ejecting from at least one of the nozzles ischanged to adjust the oxygen concentration distribution in an areawithin several millimeters of the cutting point.
 2. A laser cuttingmethod as claimed in claim 1, wherein a high oxygen concentration at thevicinity of the cutting point and a low oxygen concentration behind thecutting point in a cutting direction are maintained by ejecting a gascontaining oxygen at a high concentration from at least one of thenozzles and ejecting a gas containing oxygen at a low concentration fromthe other nozzle.
 3. A laser cutting method as claimed in claim 2,wherein the oxygen concentration of the gas containing oxygen at a highconcentration is 99.5% or more, and the oxygen concentration of the gascontaining oxygen at a low concentration is 90% or less.
 4. A lasercutting method as claimed in claim 3, wherein the gas stream of the gascontaining oxygen at a high concentration is formed around the laserbeam having a diameter in a range from 0.1 to 0.5 mm, and the gas streamof the gas containing oxygen at a low concentration is formed around thegas stream of the gas containing oxygen at a high concentration.
 5. Alaser cutting method as claimed in claim 4, wherein the lower limit ofthe oxygen concentration of the mixed gas is maintained at 50%, whenmild steel having a thickness of 15 mm or more is cut.
 6. A laser cuttercomprising a plurality of nozzle openings arranged in a line or in aring, and gas supplying means for supplying gases to the nozzle openingsand adjusting the oxygen concentration of the gases, wherein the gasesejecting from the nozzle openings can be respectively adjusted by thegas supplying means.
 7. A laser cutter as claimed in claim 6, whereinthe gas supplying means comprises a mass flow rate controller forcontrolling the mass flow rate of the oxygen.